Field of the Invention
The present invention relates to an induction type position measuring apparatus which measures a position by utilizing an induced current.
Description of the Related Art
An induction type position measuring apparatus which detects an induced current generated by electromagnetic induction and measures the position is applied to linear encoders, rotary encoders, and so forth which measure a position with high precision. The induction type position measuring apparatus has a merit that position measurement can be conducted with high precision even in a dust environment and the structure can be simplified, as compared with an optical position measuring apparatus.
The induction type position measuring apparatus includes a sensor and a scale disposed to be relatively movable along a measurement reference line. A pattern which plays a role of a coil is formed on the scale. The induction type position measuring apparatus detects an induced current which flows through the pattern on the scale by using the sensor, and performs arithmetic operation to obtain the position of the sensor relative to the scale on the basis of a detected signal.
JP 11-223505 A discloses an induction type position measuring apparatus in which periodically disposed conductor closed loops are provided on the scale and an induced current flowing through the conductor closed loops is detected by a detection wire in the sensor. Furthermore, JP 2004-003975 A discloses an induction type position measuring apparatus in which openings are provided through the scale and a position is detected by sensing an induced current flowing around the opening with the sensor. For providing the openings through the scale, a portion of a basic material which is a conductive plate material is periodically punched or removed by etching.
FIG. 10A and FIG. 10B are schematic plane views exemplifying a conventional scale. A scale 2H including one row of tracks T is exemplified in FIG. 10A. A scale 2I including three rows of tracks T1 to T3 is exemplified in FIG. 10B. By the way, in the present specification, an axis parallel to a measurement reference line ML is referred to as X axis. An axis perpendicular to the X axis and parallel to a main surface of the scale is referred to as Y axis. An axis perpendicular to the X axis and the Y axis is referred to as Z axis.
On the track T of the scale 2H illustrated in FIG. 10A, a plurality of control patterns 21 are provided at equal intervals in a direction along the X axis. In a case where a shape on a first edge portion 251 side of the control pattern 21 is different from a shape on a second edge portion 252 side of the control pattern as in the scale 2H, a route difference occurs between induced currents i1 and i1′ flowing around the control pattern 21.
Furthermore, in the scale 2I illustrated in FIG. 10B, the plurality of tracks T1 to T3 are provided in parallel. A plurality of control patterns 211 are provided on the track T1 at a period λ1 in a direction along the X axis. A plurality of control patterns 212 are provided on the track T2 at a period λ2 in a direction along the X axis. A plurality of control patterns 213 are provided on the track T3 at a period λ3 in a direction along the X axis. The periods λ1, λ2 and λ3 are different from each other.
In the scale 2I, for example, an induced current i1 flowing along the control pattern 211 on the track T1 distributes mainly around the control pattern 211 which becomes the shortest route. However, the induced current flows freely through the whole of the scale 2I. Therefore, a portion of the induced current i1 flows around the control pattern 212 on the adjacent track T2 (see induced current i2).
In the case where the scales 2H and 2I with openings provided are used as illustrated in FIGS. 10A and 10B, an induced current generated on the scale can freely through the whole region of the scale. If there is an uneven portion such as a concave shape or a convex shape on an end face on the first edge portion 251 side as in the scale 2H illustrated in FIG. 10A, the routes of the induced current i1 and i1′ are influenced. Unless an induced current flows stably for each of control pattern 21, an error occurs in detection conducted by the sensor, resulting in a problem in that high precision position measurement is obstructed.
Furthermore, in the scale 2I illustrated in FIG. 10B, a magnetic field with the period λ2 of the control pattern 212 on the track T2 is generated by the induced current i2 branched from the induced current i1, and characteristics of the position measurement is influenced.
In the case where the plurality of tracks T1 to T3 are provided in parallel as on the scale 2I, it is desirable in principle to cause the tracks T1 to T3 to be respectively independent and insulate the tracks T1 to T3 completely. In this case, however, it is necessary to manufacture a plurality of scales respectively corresponding to the tracks T1 to T3 and stick the scales together on an insulative supporting substrate. This results in a problem in that a quality degradation is caused by position deviation at the time of sticking together and the cost is increased.